Transmission type liquid crystal mask marker

ABSTRACT

The present invention relates to a transmission type liquid crystal mask marker which can be incorporated easily into a conventional apparatus, and which enables high-speed and high-accuracy stable printing and an increase in the S/N ratio of a sensor output. The transmission type liquid crystal mask marker thus includes a controller for, during rewriting a static marking image displayed on a transmission type liquid crystal mask, displaying a static test image and attaining an equipotential state, and thereafter repeatedly displaying the static marking image and attaining an equipotential state up to the end of the rewriting time, measuring an actual light transmittance of the static test image during rewriting, and regulating the voltage applied to the transmission type liquid crystal mask so that the actual light transmittance is equal to the optimum light transmittance. An optical filter can be provided on a light receiving device for measuring a light transmittance.

FIELD OF THE INVENTION

The present invention relates to a transmission type liquid crystal maskmarker, and particularly to a transmission type liquid crystal maskmarker suitable for printing on a workpiece which is required to havehigh accuracy.

BACKGROUND ART

A transmission type liquid crystal mask marker is adapted for printingvarious static images on a surface of a workpiece by applying ahigh-density energy beam, such as a laser beam, to a transmission typeliquid crystal mask (referred to as "a liquid crystal mask" hereinafterunless otherwise stated) on which the various static images aredisplayed, so as to apply to the workpiece surface the beam which haspassed through a static image. In general, only static images for a markare displayed on the liquid crystal mask of such a transmission typeliquid crystal mask marker.

One of the inventors of the present invention has previously made aproposal for maintaining the printing performance of a transmission typeliquid crystal mask marker (refer to Japanese Patent Laid-Open(a)6-39577. This proposal (referred to as "first proposal" hereinafter)has been made in consideration of the facts that the light transmittanceof a transmission type liquid crystal mask increases as the temperatureincreases and deterioration proceeds, thereby decreasing printingaccuracy, and that the light transmittance of a transmission type liquidcrystal mask can be controlled by regulating a driving voltage. In thisproposal, a light emitting element and a light receiving element areadded to a conventional transmission type liquid crystal mask marker (ifrequired, a liquid crystal mask temperature sensor is also added), andthe following program is also provided on a controller. Namely, lightfrom the light emitting element is applied to a static test image on theliquid crystal mask, and the transmitted light is received by the lightreceiving element so that an actual light transmittance Q is computedfrom the intensity of the emitted light R1 and the intensity of thereceived light R2. The applied voltage for a static image is regulatedso that the actual light transmittance Q is equal to an optimum lighttransmittance Qo at the operation temperature. It is thus possible tomaintain the printing performance of various conventional transmissiontype liquid crystal mask markers.

However, the first proposal also has a new problem with respect tomatching with a transmission type liquid crystal mask marker which hasrecently appeared and which has the performance of high-accuracy andhigh-speed continuous printing.

The assignee of the present invention has also proposed a transmissiontype liquid crystal mask marker comprising a YAG laser mask marker(refer to Japanese Patent Laid-Open (A) 5-42379). This proposal(referred to as "second proposal" hereinafter) comprises a first XYdeflecting device and a second XY deflecting device, which are providedin front of and behind a liquid crystal mask, respectively. A controllerpreviously stores each of the divided static images of the whole imagewhich is divided, and successively displays the divided static images onthe liquid crystal mask. A laser beam scans the divided static images bythe first XY deflecting device, and the laser beam passed through eachof the divided static images is deflected by the second XY deflectingdevice and applied to the workpiece surface to complete the entireprinting. As a result, high-accuracy and high-speed continuous printingis achieved.

However, a transmission type liquid crystal mask marker such as thesecond proposal, which enables high-accuracy and high-speed continuousprinting, has difficulties in timing (time matching) with the firstproposal. Namely, it is necessary to investigate a new problem withrespect to timing.

On the other hand, a transmission type liquid crystal mask marker, e.g.,a YAG laser mask marker, is sometime provided with one of varioussensors serving as a light receiving element. A silicon photodiode isfrequently used as a light receiving unit of the liquid receivingelement in consideration of the cost, quality, temperaturecharacteristics and compatibility with a light emitting element. Asshown in FIG. 6, the sensitivity A of the silicon photodiode within thewavelength region (1.06 μm) of a YAG laser is lower than the sensitivitywithin a shorter wavelength region (for example, refer to A. Yariv:Basis of Photoelectronics, P. 400, Maruzen (1988)). For example, when asemiconductor laser (wavelength of about 800 nm) or a He--Ne laser(wavelength of 633 man), which has high energy density, is used as thelight emitting element, the sensitivity of the YAG laser is as low asabout 1/10 of the sensitivity of such a semiconductor laser. Therefore,when a short-wavelength laser is used as the light emitting element, theeffects on the light receiving unit can be relatively decreased, therebypreventing the problem with respect to a S/N ratio (signal/noise ratio),even if the YAG laser beam is scattered.

However, in a transmission type liquid crystal mask marker comprising aYAG laser using a Q switch or a pulse oscillation YAG laser, a laserpulse has high peak power. Although this peak power depends upon theoperation conditions of the YAG laser or the structure of the YAG lasermask maker, the peak power is generally not less than 1 kW. Theintensity of the scattered YAG laser beam, which is applied to the lightreceiving unit, is increased depending upon the arrangement of the lightreceiving unit of the light receiving element. As a result, noise occursin the sensor output (the output of the light receiving element) due tothe YAG laser beam, and the S/N ratio of the detection system of thelight receiving element is thus decreased, thereby making unstable theoperation of the YAG laser mask marker and deteriorating printingaccuracy. In an extreme case, there are the problems that a sensingoperation is made impossible, and that an error occurs in the operationof the YAG laser mask marker.

SUMMARY OF THE INVENTION

The present invention has been achieved for solving the above problemsof the prior systems, and an object of the present invention is toprovide a transmission type liquid crystal mask marker which enables theincorporation of a program for maintaining printing accuracy into aconventional transmission type liquid crystal mask marker, and whichenables stable printing with high precision.

A transmission type liquid crystal mask marker according to the presentinvention comprises:

a light emitting means and a light receiving means which are connectedto a controller, and

a transmission type liquid crystal mask to which a voltage is appliedfrom the controller to successively rewrite and display various staticmarking images and to appropriately display a static test image;

wherein the light emitting means is a light emitting means for applyinglight to a displayed static test image;

wherein the light receiving means is a light receiving means forreceiving the light passed through the static test image;

wherein the controller is a controller for previously storing an optimumlight transmittance;

wherein, during a rewriting of at least one of the static markingimages, the displaying of the static test image is performed and then anequipotential state of all common terminals and segment terminals of thetransmission type liquid crystal mask is attainted, in that order, atleast once; and

after which the displaying of a static marking image is performed andthen an equipotential state is attained, in that order, at least once,up to the termination of the rewriting time so as to finally attain anequipotential state;

wherein the intensity of received light is inputted from the lightreceiving element during the displaying of the static test image;

wherein the actual light transmittance of the transmission type liquidcrystal mask is computed from the intensity of the received light andthe intensity of the emitted light; and

wherein a voltage for a static image applied to the transmission typeliquid crystal mask is regulated.

In addition, the image area of the static test image is larger than theincident area of the light emitted from the light emitting means, atleast on the surface of the liquid crystal mask. Further, the lightreceiving means can be a light receiving means comprising a lightreceiving unit, having a silicon pn junction for receiving the lightpassed through the static test image, and an optical filter which isprovided on a light receiving window of the light receiving unit so thatit does not transmit a YAG laser beam but transmits the above light.

In the above-described construction, during rewriting of various staticmarking images (printing images) on the liquid crystal mask, the statictest image (the image for measuring light transmittance) and each of thevarious static marking images are displayed at least once. In thedisplaying (displaying during rewriting), the actual light transmittanceof the static test image is measured and compared with the optimum lighttransmittance so as to control the voltage for a static image applied tothe liquid crystal. The applied voltage for a static image, of course,represents the applied voltage for the static test image and the appliedvoltage for a static marking image.

On the other hand, during rewriting, all common terminals and segmentterminals of the liquid crystal mask are made equipotential (referred toas "the attaining of an equipotential state" hereinafter) immediatelyafter the static test image is displayed, thereby removing substantiallyall afterimages. The displaying of the static test image and theattaining of an equipotential state are performed at least once.However, the liquid crystal mask has the characteristic that the actuallight transmittance increases as the application time of the voltage fordisplaying a static image increases. Therefore, in the presentinvention, the application of a voltage for a long time easily makesunstable the measurement of the actual light transmittance. When theattaining of an equipotential state is performed after the static testimage is displayed for a short time, therefore, a stable lighttransmittance is obtained. Even with the attaining of an equipotentialstate having the above effect, if the displaying time is increased, therising time of a static image on the liquid crystal mask is increased,thereby causing difficulties in high-speed printing.

In the present invention, therefore, after the static test image isdisplayed and an equipotential state is attained, the same static imageas a static marking image, which is displayed after the termination ofthe rewriting time, is displayed and an equipotential state is attained.The displaying of a static marking image and the attaining of anequipotential state are performed at least once up to the termination ofthe rewriting time so as to finally attain an equipotential state. As aresult, an afterimage of the static test image is securely erased, and asmooth rapid start of the displaying of the next static marking imagecan be secured after the termination of the rewriting time. The staticmarking image which is displayed during the rewriting, of course,represents an image having the same content as a first static markingimage which is displayed after the termination of the rewriting time. Itcan thus be thought that the final displayed static marking image isleft as it is, after the termination of the rewriting time, and is usedas the next static marking image. In this thinking, the rewriting timeshould be considered to start before the displaying of the final staticmarking image, i.e., at the termination of the final attaining of anequipotential state. As described above, the displaying of the statictest image and a static marking image during rewriting enablesincorporation into the first and second proposals, and the maintenanceof good printing accuracy and high printing speed.

In addition, since the image area of the static test image is largerthan the incident area of the light emitted from the light emittingmeans, the intensity of the emitted light and the intensity of thereceived light are accurately obtained, and good printing accuracy ismaintained. Further, since the optical filter is provided on the lightreceiving means so as to shield the YAG laser beam (for printing) and totransmit the light emitted from the light emitting means, the noisecaused by the YAG laser can be removed, thereby enabling high-accuracyand high-speed printing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a time chart showing the outline of a marking cycle and theoutline of the repetition of interrupt displaying in accordance with afirst embodiment of the present invention;

FIG. 2 is a flowchart in accordance with the first embodiment;

FIG. 3 is a drawing illustrating the hardware configuration of atransmission type liquid crystal mask marker in accordance with thefirst embodiment;

FIG. 4 is a time chart showing operations during a rewriting time inaccordance with the first embodiment, a second embodiment and anembodiment of another application;

FIG. 5 is a drawing illustrating the hardware configuration of atransmission type liquid crystal mask marker in accordance with a thirdembodiment of the present invention;

FIG. 6 is a graph showing the sensitivity characteristics of siliconphotodiodes in accordance with

prior systems ;

FIG. 7 is a chart showing the reflectance characteristics of an opticalfilter of a light receiving means in accordance with the thirdembodiment;

FIG. 8A is an oscilloscopic chart showing the effects of noise on thesensor output in accordance with the third embodiment; and

FIG. 8B is an oscilloscopic chart showing the effects of noise on thesensor output in accordance with prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

Transmission type liquid crystal mask markers in accordance withpreferred embodiments of the present invention are described withreference to the attached drawings.

Referring to FIG. 3, showing the hardware., configuration of a firstembodiment, the laser beam L1, which is emitted from a laser oscillator1, is first deflected to the direction Y by a galvanoscanner 9Y througha beam shaping device 81. The laser beam L1 is then deflected to thedirection X by a polygon mirror 9X and applied to a converging lens 82.The converging lens 82 converges the laser beam L1 at substantially onepoint on a reflecting mirror 83, and, at the same time, scans a staticmarking image on a transmission type liquid crystal mask 2. The,transmitted laser beam L2, which is transmitted through the transmissiontype liquid crystal mask 2 and which contains information of a staticmarking image, is deflected by a reflecting mirror 83, and is appliedthrough an objective lens 84 to a surface of a workpiece 3 on a workfeeder 10. This irradiation marks a static marking image on the surfaceof the workpiece 3. In the above-described configuration, the drivingsystems of the laser oscillator 1, the X deflecting device 9X, the Ydeflecting device 9Y, the liquid crystal mask 2, and the work feeder 10are electrically connected to the controller 7 so as to be synchronouslycontrolled by the controller 7.

A static test image, other than a static marking image, is appropriatelydisplayed on the liquid crystal mask 2 by applying a voltage from thecontroller 7. A light emitting element 5, serving as light emittingmeans, and a light receiving element 6, serving as light receivingmeans, are also provided. The light receiving element 6 receives thelight emitted from the light emitting element 5 and passed through thestatic test image, which is displayed on the liquid crystal mask 2, andoutputs to the controller 7 the intensity of the received light R2. Itis thus preferable that the area of the static test image is larger thanthe incident area of the light emitted from the light emitting element5, at least on the surface of the liquid crystal mask 2. The lightemitting element 5 and the light receiving element 6 are electricallyconnected to the controller 7 to be synchronously controlled by thecontroller 7. The controller 7 previously stores an optimum lighttransmittance Qo and the intensity of the emitted light R1, emitted fromthe light emitting element 5. The optimum light transmittance Qo is avalue which is previously stored in the controller 7 on the basis of themeasurements of the characteristics of the transmission type liquidcrystal mask 2 of the transmission type liquid crystal mask marker inordinary use.

The software configuration of the first embodiment is described below.As shown in FIG. 1, this embodiment relates to a transmission typeliquid crystal mask marker of high-accuracy and high-speed continuousprinting type having a printing cycle t of about 0.8 second. Theprinting cycle t includes a displaying time (printing time) t1 of astatic marking image of about 0.3 second, and a workpiece standby time(the time of movement and fixing at the printing position of aworkpiece) t2 of about 0.5 second. In the workpiece standby time t2,i.e., the rewriting time t2, the interrupt displaying time t21 of thestatic test image or a static marking image is about 0.1 second. Thetime for making 0 v all common terminals and segment terminals of theliquid crystal mask 2 (i.e., grounding all terminals), or the time t22for attaining an equipotential state after this interrupt displaying isabout 0.04 second. In this embodiment, the displaying and the attainingof an equipotential state are repeated three times within the workpiecestandby time t2. FIG. 2 is a flowchart showing the repetition.

In the displaying and the attaining of an equipotential state in thefirst embodiment, the controller 7 first displays the static test imageduring the rewriting time t2, as shown in FIG. 4. The controller 7inputs the intensity of the received light R2 from the light receivingmeans 6 during the displaying time t21 of about 0.1 second. The actuallight transmittance of the transmission type liquid crystal mask 2 iscomputed from the intensity of the received light R2 and the intensityof the emitted light R1 of the light emitting means 5. The controller 7controls the voltage for a static image applied to the liquid crystalmask 2 so that the actual light transmittance Q is equal to the optimumlight transmittance Qo. After the elapse of the displaying time t21, allterminals are made 0 V to attain an equipotential state. After thisattaining of an equipotential state, the same static image as the firststatic marking image after the termination of the rewriting time t2 isdisplayed, and then an equipotential state is attained. After thisattaining of an equipotential state, the elapsed time t from the startof rewriting is compared with the rewriting time t2. If t <t2, thedisplaying of the static test image, and the attaining of anequipotential state are repeated. If t=t2, i.e., if it is decided thatthe rewriting time is terminated, the next regular static marking imageis displayed and printed.

Since the liquid crystal mask 2 is controlled to the optimum lighttransmission Qo by the above-described displaying and the attaining ofan equipotential state, the next regular static marking image is printedon the work surface 3 with high accuracy. Since afterimages of theregular static marking image and the static test image are securelyerased before the start of rewriting, the displaying of a regular staticmarking image after the end of rewriting can rapidly and smoothly bestarted. Although, in this embodiment, a static image is displayed threetimes, the static image can be displayed any desired number of times inaccordance with the specifications of the body of the transmission typeliquid crystal mask marker used.

A second embodiment of the present invention is described. In thisembodiment, unlike the first embodiment, the displaying of the statictest image and the attaining of an equipotential state are repeated aplurality of times. Drawings similar to FIGS. 1 and 3 of the firstembodiment are not described.

In FIGS. 3 and 4, the controller 7 first displays the static test imageand inputs the intensity of the received light R2 during the rewritingtime t2. An equipotential state is then attained, and the number oftimes n that the static test image has been displayed is then comparedwith the predetermined number of times N that the static test image isto be displayed. If n<N, the displaying of the static test image and theattaining of an equipotential state are repeated. If n=N, the samestatic image as the first static marking image after the end of therewriting time t2 is displayed. After this displaying, an equipotentialstate is finally attained at the end of the rewriting time t2, as in thefirst embodiment.

The second embodiment enables the erasing of afterimages, thestabilization of the intensity of the received light R2, and a morerapid and smoother start of the displaying of the next regular staticmarking image. Although, in this embodiment, the number of times astatic image is displayed is three, the number of times of displayingcan be set to any number according to demand. For example, if the numberof times of displaying is five, the two times of earlier displaying canbe performed for the static test image, and the three times of laterdisplaying can be performed for a static marking image. It is a matterof course that the number of times of earlier displaying and the numberof times of later displaying can be reversed. Although the static testimage is displayed a plurality of times N, the intensity of the receivedlight R2 can be inputted to the controller 7 at least once.

As is obvious from the first and second embodiments, when the controllerhaving the aforementioned program is applied to a conventionaltransmission type liquid crystal mask marker, a program for maintainingprinting accuracy can be incorporated without any difficulties. There isthus no need to change the configuration of the bodies of thetransmission type liquid crystal mask marker and the work feeder. Inaddition, since the static test image is displayed at a high speed, eachof the embodiments is a help to the increasing of the printing speed andthe printing accuracy by increasing the operation speed of theconventional transmission type liquid crystal mask marker and the workfeeder body. As a result, it is possible to maintain and further improvethe high printing speed and printing accuracy, thereby permitting anattempt to increase the throughput.

Another embodiment proposed by the assignee of the present invention isdescribed in international application No. PCT/JP94/00703. Thisembodiment has the same hardware configuration as the first embodimentof the present invention, but the software configuration is different inthe displaying of a static image and the attaining of an equipotentialstate during the rewriting time. Namely, as shown in FIG. 4, the statictest image is displayed at the start of rewriting, the intensity of thereceived light R2 is inputted from the light receiving means 6, and thevoltage for a static image to be applied to the liquid crystal mask 2 isregulated so that the actual light transmittance Q is equal to theoptimum light transmittance Qo. After the static test image isdisplayed, an equipotential state is attained, and the displaying of thestatic test image and the attaining of an equipotential state arerepeated up to the termination of the rewriting time t2. As a result,any afterimages are erased, the intensity of the received light R2(i.e., the actual light transmittance Q) is made stable, and thedisplaying of the next regular static marking image for printing isstarted at high speed.

In PCT/JP94/00703, the displaying of the static test image and theattaining of an equipotential state are repeated. However, in thepresent invention, after the displaying of the static test image and theattaining of an equipotential state are performed once, the displayingof a static marking image and the attaining of an equipotential stateare repeated at least once. Since, in the present invention, the statictest image and a static marking image are displayed, the start of aregular static marking image for printing is further improved, ascompared with the PCT/JP94/00703.

A transmission type liquid crystal mask marker in accordance with athird embodiment of the present invention is described below withreference to the attached drawings.

FIG. 5 shows the hardware configuration of a transmission type liquidcrystal mask marker comprising a YAG laser with a Q switch in accordancewith the third embodiment. Points different from FIG. 1 of the firstembodiment are described. The third embodiment is different from thefirst embodiment in the points that a deflecting lens 86 is providedbetween the galvanoscanner 9Y and the polygon mirror 9X, and that anoptical filter 61 is provided on a light receiving window of the lightreceiving means 6, serving as the light receiving element. When thedeflecting lens 86 is provided, the laser beam L1, deflected to thedirection Y by the galvanoscanner 9Y, is deflected by the deflectinglens 86 and applied to substantially the same point on the polygonmirror 9X Since the irradiation point can be controlled within a narrowrange, the thickness of the polygon mirror 9X can be decreased. It isthus possible to decrease the weight of the polygon mirror 9X, increasethe operation speed of the polygon mirror 9X, and decrease the sizethereof. The basic function is similar to the transmission type liquidcrystal mask marker (refer to FIG. 1) without the deflecting lens 86.The laser beam L1 applied to the polygon mirror 9X is passed through theliquid crystal mask 2 for displaying a static marking image to print theimage on the workpiece surface 3 at a high speed with high accuracy, asin the first embodiment.

The light receiving means 6 provided with the optical filter 61 isdescribed below. The light receiving means 6 receives the transmittedlight emitted from the light emitting means 5 as the light emittingelement, and outputs the intensity of the received light R2 to thecontroller 7. The controller 7 computes the actual light transmittance Qfrom the intensity of the received light R2 and the intensity of theemitted light R1, and controls the voltage for a static image applied tothe liquid crystal 2 so that the actual light transmittance Q equals tothe optimum light transmittance Qo. As a result, constant image printingquality is maintained.

The light emitting means 5 comprises a semiconductor laser whichoscillates with a wavelength of 780 nm, the light receiving unit of thelight receiving means 6 comprises a silicon photodiode, and the opticalfilter 61 comprises a YAG laser deflecting filter. As shown in FIG. 7,the reflectance of the optical filter 61 within the wavelength region ofthe YAG laser is not less than 90% and the reflectance within thewavelength region of the semiconductor laser is substantially 0. Namely,the optical filter 61 does not transmit the YAG laser beam L(transmitted laser beam L2), but transmits the light R for measuring thelight transmittance (the light with the intensity of the received lightR2). The optical filter 61 is generally referred to as an "interferencefilter".

FIGS. 8A and 8B show data examples of the sensor output of thisembodiment, provided with the optical filter 61, and the prior system(without the optical filter 61). Since the sensor output B (priorsystem) contains the large noise caused during emission of the YAG laserbeam L, the output value is unstable, and the sensor output B is thusunsuitable for high-accuracy control. This is due to the noise caused byQ switch control of the YAG laser. However, noise is completely removedfrom the sensor output C of this embodiment, and the intensity of thereceived light R2 from the light emitting means 5 is stable. Thetransmission type light crystal mask marker is stably operated by thedriving control based on the sensor output C with an excellent S/Nratio, thereby preventing operation error. The image marking quality canalso be maintained in a high level. In addition, although the priorsystem has the need to decrease the effect of noise by disposing theoptical sensor (the light receiving element) at a position where the YAGlaser beam is hardly applied, this embodiment eliminates such a limitand thus increases the degree of freedom of arrangement.

INDUSTRIAL APPLICABILITY

The present invention can easily be applied to a conventional apparatusby only changing a program of the controller, and enables high-speed andhigh-accuracy stable printing, by smoothly starting the displaying of animage for printing while controlling the light transmittance of theliquid crystal mask to an optimum state, and an attempt to increase thethroughput. The present invention is also useful as a transmission typeliquid crystal mask marker which can remove noise by an optical filterto increase the S/N ratio of sensor output.

That which is claimed is:
 1. A transmission liquid crystal mask markercomprising:a laser oscillator for providing a laser beam; a transmissionliquid crystal mask adapted for successively rewriting and displayingvarious static marking images and for appropriately displaying a statictest image in response to an applied voltage, wherein said transmissionliquid crystal mask has common terminals and segment terminals; and acontroller connected to said transmission liquid crystal mask forapplying said voltage to said transmission liquid crystal mask fordisplaying the various static marking images so that the various staticmarking images are printed on a surface of a workpiece by applying thelaser beam, which has been transmitted from said laser oscillatorthrough said transmission liquid crystal mask onto the surface of saidworkpiece; a light emitter connected to said controller and adapted toapply light to the static test image; and a light receiver connected tosaid controller and adapted to receive the light transmitted from thelight emitter through the static test image, wherein said controller isadapted for previously storing an optimum light transmittance; wherein,during a rewriting time for rewriting at least one of the various staticmarking images, said controller causes the displaying of a static testimage and then causes all common terminals and segment terminals of saidtransmission liquid crystal mask to attain an equipotential state, inthat order at least once, wherein, after the displaying of a static testimage and the attainment of an equipotential state at least once, saidcontroller causes the displaying of a static marking image and thencauses all common terminals and segment terminals of said transmissionliquid crystal mask to attain an equipotential state, in that order atleast once, up to a termination of the rewriting time; wherein theintensity of received light is inputted by said light receiver to saidcontroller during the displaying of the static test image; wherein saidcontroller computes the actual light transmittance of said transmissionliquid crystal mask from the intensity of the received light and theintensity of the emitted light emitted by said light emitter; andwherein said controller regulates said voltage so that the actual lighttransmittance equals the stored optimum light transmittance.
 2. Atransmission liquid crystal mask marker in accordance with claim 1,wherein, during a rewriting time for rewriting at least one of thevarious static marking images, said controller is adapted to cause thedisplaying of a static test image and then cause all common terminalsand segment terminals of said transmission liquid crystal mask to attainan equipotential state, repeatedly in that order for a plurality oftimes.
 3. A transmission liquid crystal mask marker in accordance withclaim 1, wherein, after the displaying of a static test image and theattainment of an equipotential state at least once, said controller isadapted to cause the displaying of a static marking image and then causeall common terminals and segment terminals of said transmission liquidcrystal mask to attain an equipotential state, in that order for aplurality of times, up to a termination of the rewriting time.
 4. Atransmission liquid crystal mask marker in accordance with claim 3,wherein, during a rewriting time for rewriting at least one of thevarious static marking images, said controller is adapted to cause thedisplaying of a static test image and then cause all common terminalsand segment terminals of said transmission liquid crystal mask to attainan equipotential state, repeatedly in that order for a plurality oftimes.
 5. A transmission liquid crystal mask marker in accordance withclaim 4, wherein said laser oscillator provides a YAG laser beam; andwherein said light receiver comprises a light receiving unit having asilicon pn junction, for receiving light transmitted through the statictest image, and an optical filter, which is provided on a lightreceiving window of said light receiving unit so that said opticalfilter does not transmit said YAG laser beam but transmits the lighttransmitted through the static test image.
 6. A transmission liquidcrystal mask marker in accordance with claim 4, wherein an image area,on the surface of said transmission liquid crystal mask, of the statictest image is larger than an incident area of light emitted from saidlight emitter.
 7. A transmission liquid crystal mask marker inaccordance with claim 1, wherein an image area, on the surface of saidtransmission liquid crystal mask, of the static test image is largerthan an incident area of light emitted from said light emitter.
 8. Atransmission liquid crystal mask marker in accordance with claim 7,wherein said laser oscillator provides a YAG laser beam; and whereinsaid light receiver comprises a light receiving unit having a silicon pnjunction, for receiving light transmitted through the static test image,and an optical filter, which is provided on a light receiving window ofsaid light receiving unit so that said optical filter does not transmitsaid YAG laser beam but transmits the light transmitted through thestatic test image.
 9. A transmission liquid crystal mask marker inaccordance with claim 1, wherein said laser oscillator provides a YAGlaser beam; and wherein said light receiver comprises a light receivingunit having a silicon pn junction, for receiving light transmittedthrough the static test image, and an optical filter, which is providedon a light receiving window of said light receiving unit so that saidoptical filter does not transmit said YAG laser beam but transmits thelight transmitted through the static test image.
 10. A method forprinting a mark on a surface of a workpiece, said mark being formed froma succession of a plurality of static marking images, said methodcomprising the steps of:storing an optimum light transmittance;directing a laser beam toward a transmission liquid crystal mask havingcommon terminals and segment terminals; applying a voltage to saidtransmission liquid crystal mask for successively rewriting anddisplaying various static marking images so that the laser beam isselectively transmitted by the transmission liquid crystal mask ontosaid surface of said workpiece to thereby print the various staticmarking images on said surface; during a time of rewriting of at leastone of said various static marking images:applying said voltage to saidtransmission liquid crystal mask for appropriately displaying a statictest image in response thereto, and then causing all of said commonterminals and segment terminals to attain an equipotential state, inthat order at least once; applying light emitted from a light emitter tothe thus displayed static test image; detecting light transmitted fromsaid light emitter through the thus displayed static test image; andafter the displaying of a static test image and the attainment of anequipotential state at least once, causing the displaying of a staticmarking image and then causing all of said common terminals and segmentterminals of said transmission liquid crystal mask to attain anequipotential state, in that order at least once, up to a termination ofthe time of rewriting; calculating an actual light transmittance of saidtransmission liquid crystal mask from an intensity of the thus detectedlight and an intensity of the light emitted by said light emitter; andregulating said voltage so that the actual light transmittance equalsthe thus stored optimum light transmittance.
 11. A method in accordancewith claim 10, wherein the thus displayed static test image has an imagearea, on the surface of said transmission liquid crystal mask, which islarger than an incident area of light emitted from said light emitter.12. A method in accordance with claim 10, wherein said laser beam is aYAG laser beam; and wherein said method further comprises filtering thelight transmitted through the static test image so as to transmit thelight from the light emitter while excluding said YAG laser beam fromthe step of detecting light transmitted from said light emitter throughthe thus displayed static test image.
 13. A method in accordance withclaim 10, wherein said steps of causing the displaying of a staticmarking image after the displaying of a static test image and theattainment of an equipotential state and then causing all of said commonterminals and segment terminals of said transmission liquid crystal maskto attain an equipotential state comprise displaying the next one ofsaid various static marking images in the succession to be printed onsaid surface and then causing all of said common terminals and segmentterminals of said transmission liquid crystal mask to attain anequipotential state.
 14. A method in accordance with claim 10, whereinsaid steps of causing the displaying of a static marking image after thedisplaying of a static test image and the attainment of an equipotentialstate and then causing all of said common terminals and segmentterminals of said transmission liquid crystal mask to attain anequipotential state are repeated, in that order, a plurality of timesduring the time of rewriting.
 15. A method in accordance with claim 10,wherein said steps of applying said voltage to said transmission liquidcrystal mask for appropriately displaying a static test image inresponse thereto, and then causing all of said common terminals andsegment terminals to attain an equipotential state, are repeated in thatorder a plurality of times during the time of rewriting.
 16. A method inaccordance with claim 15, wherein said steps of causing the displayingof a static marking image after the displaying of a static test imageand the attainment of an equipotential state and then causing all ofsaid common terminals and segment terminals of said transmission liquidcrystal mask to attain an equipotential state are repeated, in thatorder, a plurality of times during the time of rewriting.
 17. A methodin accordance with claim 16, wherein said steps of causing thedisplaying of a static marking image after the displaying of a statictest image and the attainment of an equipotential state and then causingall of said common terminals and segment terminals of said transmissionliquid crystal mask to attain an equipotential state comprise displayingthe next one of said various static marking images in the succession tobe printed on said surface, and then causing all of said commonterminals and segment terminals of said transmission liquid crystal maskto attain an equipotential state.
 18. A method in accordance with claim17, wherein the thus displayed static test image has an image area, onthe surface of said transmission liquid crystal mask, which is largerthan an incident area of light emitted from said light emitter.
 19. Amethod in accordance with claim 17, wherein said laser beam is a YAGlaser beam; and wherein said method further comprises filtering thelight transmitted through the static test image so as to transmit thelight from the light emitter while excluding said YAG laser beam fromthe step of detecting light transmitted from said light emitter throughthe thus displayed static test image.
 20. A method in accordance withclaim 19, wherein the thus displayed static test image has an imagearea, on the surface of said transmission liquid crystal mask, which islarger than an incident area of light emitted from said light emitter.